How can Europe, occupied Korea, or Japan credibly think that the US will defend them at that point?

It's disturbing if they currently think US will defend them, when all defenses are moved to Israel today, and Trump shifts from "not our problem, we should leave" and "permanent global economy destruction", and delaying "permanent global destruction" while Israel pursues it anyway. Furthermore, pursuing the Israel genocide expansion plan makes US angry at low enthusiasm level of its colonies to join, and promises "bad future" for its allies.

If they’re forced to abandon all their vassals in the Gulf, then their image as a world power collapses overnight.

That is most likely outcome. Of the $200B funding request bill, as much as $50B is expected as earmark to allies reparations other than Israel. Unclear that congress will go along with any funds for them.